Idiom：鹬蚌相争，渔翁得利（yù bàng xiāng zhēng, yú wēng dé lì）
It means that it's the third party that benefits from the tussle.
Followed is a story which might help you understand it.
In China, there is a very famous TV drama named Nirvana in Fire. One part of the TV serial, it told about the story between two sons of Emperor. They both wanted to be master of the capital of country, they always brawled with each other in front of their father Emperor.
Emperor was so confused about dealing with it.
Under this situation, different ministers supported different sons of Emperor. So Emperor still didn’t make a decision.
Unexpectedly finally Emperor chose his other son named Mr. Jing to be the master of the capital of one country. This son went on an expedition all years and he was good at managing the army. This duty for him was a piece of cake.
Because the quarrel between two sons made Emperor cheesed, he suddenly remembered his other son Mr. Jing. Although generally he didn’t pay any attention to him, at last gave the job to Mr. Jing.
Here Mr. Jing is the third party that benefits from the tussle.
成语：鹬蚌相争，渔翁得利（yù bàng xiāng zhēng, yú wēng dé lì）
解释：比喻双方相持不下，而使第三者从中得利。 语出清•湘灵子《轩亭冤•哭墓》。
电视剧《琅琊榜》的内容很精彩，其中有一段是太子和誉王，是当朝两位最有权势的皇子，在争夺京城巡防营（相当于如今的公安局）的控制权，两人日日在圣驾之前争吵不休，让皇上也十分的困扰。
朝中的大臣们也各自为营，辅助自己推崇的主子象征这个差事。皇上敏感多疑，迟迟没有做出决策。
而两皇子苦苦相争的局面中，皇上突然把巡防营的控制交给了并不算十分受宠的靖王管理，靖王常年征战，善于管理军队，接收一个巡防营，并不算什么难事。只是当朝掌权的两位皇子相争不下，让皇上十分厌恶之时，方想起把这个差事交给并不算十分受重视的靖王管理。靖王可谓是“鹬蚌相争，渔翁得利”中的渔翁了。
Author: @linda65
Translator: @sunnyjolly
向翻译官致敬，o(￣︶￣)o
么么哒